This invention is directed generally to systems that provide automatic gain control for speech signals. In particular, this invention is directed to improving the performance of automatic gain control systems that depend upon the detection of speech signals to function properly, such the automatic level control (ALC) circuit of U.S. Pat. No. 4,514,703 issued to Maher et al. This circuit may also have applications wherever speech detection is required.
Conventional AGC systems produce a nearly constant amplitude output speech signal as input speech signals vary in amplitude. However, conventional AGC circuits increase gain when the input signal ceases because there is no speech present upon which the AGC system can operate. In most audio systems, the only signal present in the absence of a speech signal, is background noise. Since there is no signal present, the background noise is amplified by the audio system as the AGC it increases its gain.
To overcome the limitations with conventional AGC systems, some AGC systems incorporate an automatic level control, or ALC, that detects speech and allows the AGC system gain to vary freely in the presence of a speech signal. In the absence of a speech signal, the ALC circuitry, which generally includes speech detection circuits, hold the system gain constant until a speech signal reappears at the AGC circuit input. A prior art AGC system including an ALC, can be seen in U.S. Pat. No. 4,541,703. The ALC system disclosed in this patent includes a speech detector that holds the gain adjustment of the AGC circuitry constant when speech is not present at the audio input terminal.
A problem with the ALC of Maher, however, is its inability, under certain operating conditions, to distinguish between certain types of background noise and actual speech. In particular, it was found that vehicular noise may be confused with speech, resulting in periods of continuous AGC operation following pauses in speech. This circuit produced changes in gain that resulted in the occurrence of loud intersyllabic noise bursts as the AGC increased the gain level in response to background noise.
An ALC circuit that can detect speech more reliably and which could be used to control the gain of an AGC circuit would be an improvement over the prior art. Such circuit would differentiate between background noise more reliably, detecting speech peaks and background noise levels to control AGC circuitry.